The present invention relates generally to polycrystalline silicon and more specifically to a method of fabricating thin polycrystalline silicon fuses.
Generally, two materials have been used to form fuses in integrated circuits. These materials are generally nichrome and polycrystalline silicon. The method of control of nichrome thickness is a control crystal in the deposition machine. The frequency of the crystal lowers with increased nichrome thickness. The method of control of polycrystalline fuse thickness has been to use an intensity-versus-wavelength thickness measurement tool to measure the thickness of the polycrystalline layer over a controlled oxide on a product or pilot wafer after deposition. In addition, thicknesses can be measured on a calibrated S.E.M. or the film patterned and then measured on a stylus having calibrated amplified wafer-normal displacements.
For certain integrated circuits, for example, metal oxide insulated gate field effect transistors, nichrome and other similar type of materials have not been considered compatible with processing techniques. For these integrated circuits, the polycrystalline silicon fuse is preferred.
In bipolar as well as insulated gate field effect transistor circuits, the current limiting impedance in the fuse path as well as the characteristics of the fuse itself set how much power can be delivered to the fuse during fusing. Because of the circuit layouts, some integrated circuits have a significant number of hard-to-program fuses. To resolve this problem, the fuse has to see more power. Instead of redesigning the circuit to match the fuse characteristics, it is preferable to redesign the fuse to match the circuit limitations. Tests have indicated that the narrower and the thinner the fuse, the better the fuse programmability. To design fuses which will program reliably, the thickness of the polycrystalline layer must be less than 1000 Angstroms. Realizing that polycrystalline cannot be deposited uniformly without pinholes and other undesirable structural defects below 1200 Angstroms, a method of thinning polycrystalline material controlably must be found.
An object of the present invention is to provide a method of fabricating thin film polycrystalline fuses of high reliability.
Another object of the present invention is to provide a method of fabricating specific resistance polycrystalline fuses of approximately 400 to 1000 ohm resistance.
Still another object of the present invention is to provide a method of fabricating thin film, low resistance polycrystalline fuses with very high probability of programming with a limited number of pulses programming algorithm.
These and other objects of the invention are attained by depositing a layer of polycrystalline silicon at a controlled thickness of greater than 1200 Angstroms, oxidizing the polycrystalline layer to form a polycrystalline layer of a thickness below 1000 Angstroms with an oxide layer thereon, removing oxide over sample fuses and measuring the thickness of the polycrystalline layer. The final desired thickness is below 950 Angstroms. This method assures uniform thin layers of polycrystalline material to accurate thicknesses which results in desired resistances. The polycrystalline layer is oxidized in a partial oxygen rich environment, for example, steam or oxygen wherein the oxygen rich environment represents less than 50% of the total environment. The polycrystalline layer may be doped with impurities to lower its resistance. Similarly, the polycrystalline layer may be patterned to the fuse geometry before or after the oxidation step.
The formation of oxide layers on polycrystalline material is well known in the prior art. As described in U.S. Pat. No. 3,792,319 to Tsang, the polycrystalline layer is partially oxidized to form a thick mask layer to be used for patterning the polycrystalline layer. Although the polycrystalline layer is thinned as a byproduct of this operation, the importance of the step is the thickness of the oxide layer such that it can operate as a mask layer. Thus, this process is optimized for the oxide layer and not the resulting polycrystalline layer. The prior art shows other examples of partially or fully oxidizing a polycrystalline layer to produce a desired oxide thickness for a specific purpose. Prior to the present invention, the prior art failed to control the oxidation process to produce a desired thickness of polycrystalline layer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.